Recently, it has been investigated to use an electrochemical device and a power generator in combination as the power source for a portable electronic device, such as an information device, a communication device, a visual device, an audio device, or a still picture device, or an electric vehicle. When an electrochemical device and a power generator are used in combination, if the amount of power generated by the power generator is insufficient, power can be supplied from the electrochemical device to a load device, and if the amount of power generated by the power generator is excessive, the excessive power can be charged into the electrochemical device.
However, if the remaining capacity of the electrochemical device is small, when the amount of power generated by the power generator is insufficient, the electrochemical device cannot supply a necessary amount of power to the load device, so that the operation of a portable device or an electric vehicle becomes unstable. Also, if the remaining capacity of the electrochemical device is excessive, when the amount of power generated by the power generator is excessive, all the excessive power cannot be charged into the electrochemical device, so that energy is wasted. Therefore, in a power system including an electrochemical device, a load device, and a power generator, it is desired to constantly monitor the remaining capacity of the electrochemical device, and to keep the remaining capacity of the electrochemical device above a certain level.
Referring now to FIG. 1, a description is given of a method for monitoring the remaining capacity of an electrochemical device in a conventionally proposed power system. FIG. 1 is a block diagram of a conventional power system (see Patent Document 1). The system of FIG. 1 has a fuel cell 30 as a power generator that supplies power to a load device 5. This system has a storage battery 60 as a back-up electrochemical device. Fuel produced by a reformer 20 is supplied to the fuel cell. A raw material of fuel is supplied to the reformer 20 from a raw material tank 101.
The output of the fuel cell 30 is produced by controlling various auxiliaries by means of an auxiliary controller 100. That is, the auxiliary controller 100 controls a reaction air blower 90 that supplies air to the cathode of the fuel cell, a combustion air blower 80 that supplies air to the burner of the reformer 20, and a raw material pump 70 that supplies the raw material of fuel from the raw material tank 101 to the reformer. The value of current outputted from the fuel cell is detected by an output current detector 170.
Meanwhile, the storage battery 60 is connected to a current detector 110 and a voltage detector 130. The current detected by the current detector 110 is integrated by an integrator 120. The detected voltage and the integrated current value are sent to a system controller 150. The system controller 150 is equipped with a memory 140, and the memory stores the target value of remaining capacity of the storage battery. The system controller 150 performs a calculation based on the detected voltage value, the integrated current value, the target value of remaining capacity, and the like, and controls the auxiliary controller 100 based on the calculated result. The current value detected by the output current detector 170 and the calculated result produced by the system controller 150 are sent to a data comparator 180, where they are compared with each other. Based on the compared result, a regulator 160, which controls a DC/DC converter 40, is controlled.
The above-described system can control the current supplied from the fuel cell to the storage battery and the load based on the remaining capacity of the storage battery. However, a circuit equipped with the current detector 110 and the integrator 120 is necessary, and the control method becomes complicated. The current detector is composed of a shunt resistor, a hall element or the like and thus costly.
FIG. 2 shows a charge/discharge curve of a common electrochemical device.
As shown in FIG. 2, the charge/discharge curve can be divided into three regions A to C. In the region B, a reversible charge/discharge reaction proceeds in the electrochemical device, but in the region C, there is a tendency of poor reversibility. It is thus preferred to utilize only the flat region B exhibiting stable output in a power system including an electrochemical device, a load device, and a power generator.
However, in the flat region B, the voltage hardly changes, so it is difficult to utilize only the voltage in order to monitor the remaining capacity of the electrochemical device. Also, if conditions such as a current value and ambient temperature are different, different voltage values are detected for the same remaining capacity. Further, even if the voltage of an electrochemical device is estimated by utilizing a parameter such as a current value, this is not useful for estimating the remaining capacity of the electrochemical device in the flat region of the charge/discharge curve. Accordingly, it is necessary to employ a method of integrating current values with respect to time (hereinafter referred to as current integration) to monitor the remaining capacity.
Also, when the B region is not completely flat and its inclination is gentle, utilizing only the voltage for monitoring the remaining capacity of an electrochemical device requires a high level of voltage detection accuracy. It also requires accuracy in detecting a parameter necessary for correction. However, trying to achieve such accuracy results in high costs. On the other hand, if the accuracy is poor, it is not possible to correctly monitor the remaining capacity based on the voltage. Therefore, current integration eventually becomes necessary to correctly determine the remaining capacity of the electrochemical device.
Meanwhile, if an electrochemical device is fully charged (until the remaining capacity becomes 100%) and the integrated current value is reset to 100%, this reset eliminates integration errors. Performing this operation periodically makes it possible to monitor the remaining capacity with relatively good accuracy.
In the case of the system of FIG. 1, when the electrochemical device is in a fully charged state or a fully discharged state, if the system controller 150 is caused to recognize the remaining capacity of the electrochemical device as 100% or 0%, the remaining capacity can be reset. With the remaining capacity after reset being stored, by starting integration of charge/discharge current of the electrochemical device, the measurement accuracy of the remaining capacity can be heightened.
It should be noted, however, that the amount of power consumed by the load device (energy demand) may vary constantly and is unpredictable. The operation of the load device may abruptly stop. In this case, if the electrochemical device is fully charged to make the remaining capacity to 100%, the electrochemical device is unable to absorb power generated by the power generator, so that the energy is lost. Such energy loss is particularly significant in a system including a power generator, such as a fuel cell, which needs considerable time to stop. Conversely, when the electrochemical device is fully discharged until the remaining capacity becomes 0% in order to reset the integrated current value, it is unable to cope with a rapid increase in the amount of power consumed by the load device. Also, if the system is stopped with the remaining capacity being 0%, the system may be unable to be started the next time.
For example, in the case of electric vehicles and hybrid vehicles, a full charge of the electrochemical device is avoided such that the electrochemical device can collect regenerative energy produced when the vehicle is decelerated. Also, a full discharge of the electrochemical device is avoided such that the electrochemical device can make up for a shortage of energy needed upon acceleration. In this way, the system continues to operate based on current integrals, voltage values and the like, without being able to make the remaining capacity to 100% or 0% to reset the remaining capacity.
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 1-211860 (FIG. 1 and FIG. 3)